1. Field of the Invention
The present invention relates to a power unit mounted on a vehicle such as a three-wheeled automotive vehicle and including an internal combustion engine and a power transmitting device for transmitting power of the internal combustion engine to a drive wheel. More particularly, the present invention relates to an arrangement of the internal combustion engine and a pivot portion for swingably supporting the power unit to a vehicle body.
2. Description of Background Art
A conventional vehicle power unit includes an internal combustion engine and a power transmitting device which is supported on a vehicle body at a front pivot portion and a rear pivot portion spaced from each other in a longitudinal direction of the vehicle body so as to be vertically swingable about the front pivot portion.
For example, a power unit mounted on a motorcycle described in Japanese Utility Model Publication No. Sho 62-23349 includes an internal combustion engine having a cylinder portion inclined down to the rear side from a vertical direction. The cylinder portion and a crankshaft of the engine are located above a plane containing the axes of a front pivot portion and a rear pivot portion for pivotably supporting the power unit to a vehicle body.
Another power unit mounted on a three-wheeled automotive vehicle described in Japanese Utility Model Laid-open No. Sho 62-54891 includes an internal combustion engine having a cylinder portion inclined down to the front side from a vertical direction. A crankshaft of the engine is located below a plane containing the axes of a front pivot portion and a rear pivot portion for pivotably supporting the power unit to a vehicle body.
In the former prior art power unit, the crankshaft and the cylinder portion of the engine are located above the plane, and the cylinder portion largely projects upward from the plane. Accordingly, the moment of inertia of the cylinder portion about a straight line lying on the plane and connecting the lateral center of the front pivot portion and the lateral center of the rear pivot portion is large. As a result, a large moment about the straight line acts on the front pivot portion and the rear pivot portion of the power unit because of vibrations of the cylinder portion by the combustion in the engine. Therefore, the power unit including the cylinder portion tends to vibrate about the straight line, causing the generation of noise.
In the latter prior art power unit, the crankshaft of the engine is located below the plane, and the cylinder portion is inclined down to the front side from the vertical direction. Accordingly, the cylinder portion largely projects upward from the plane. Therefore, the power unit including the cylinder portion tends to vibrate as in the former case, causing the generation of noise.
It is accordingly an object of the present invention to suppress vibrations of a power unit which is vertically swingably supported to a vehicle body, thereby suppressing the generation of noise.
In accordance with a first aspect of the present invention, there is provided a vehicle power unit including an internal combustion engine having a crankshaft extending in a lateral direction of a vehicle body and a cylinder portion having an axis which is inclined down to the rear side from a vertical direction, and a power transmitting device for transmitting power of the internal combustion engine to a rear wheel located behind the crankshaft, the vehicle power unit being supported to the vehicle body at a front pivot portion and a rear pivot portion spaced from each other in a longitudinal direction of the vehicle body so as to be vertically swingable about the front pivot portion; wherein the crankshaft and an axle of the rear wheel are located below a plane containing the axes of the front pivot portion and the rear pivot portion, and the plane and the cylinder portion intersect each other between the front pivot portion and the rear pivot portion as viewed in side elevation.
With this arrangement, the plane is located above the crankshaft and the axle of the rear wheel, and the plane and the cylinder portion having an axis which is inclined down to the rear side intersect each other between the front pivot portion and the rear pivot portion. Accordingly, the amount of upward projection of the cylinder portion from the plane can be reduced.
As a result, it is possible to reduce the moment of inertia of the cylinder portion about a straight line lying on the plane and connecting the lateral center of the front pivot portion and the lateral center of the rear pivot portion. Therefore, it is possible to reduce the moment acting on the front pivot portion and the rear pivot portion due to vibrations of the cylinder portion by the combustion in the internal combustion engine. Accordingly, vibrations of the power unit including the cylinder portion can be suppressed to thereby suppress the generation of noise.
In accordance with a second aspect of the present invention, the power transmitting device includes a transmission having an output shaft geared to the axle of the rear wheel, the output shaft being located below a part of the cylinder portion so as to be vertically overlapped and below a plane containing the axis of the crankshaft and the axis of the axle of the rear wheel.
With this arrangement, the output shaft of the transmission geared to the axle of the rear wheel is located below the plane containing the axis of the crankshaft and the axis of the axle of the rear wheel. Accordingly, the cylinder portion vertically overlapped with the output shaft can be further inclined down to the rear side without the interference with the output shaft.
As a result, it is possible to increase the degree of freedom of tilt arrangement of the cylinder portion for reducing the amount of upward projection of the cylinder portion from the plane containing the axis of the front pivot portion and the axis of the rear pivot portion. Accordingly, optimum arrangement of the cylinder portion with respect to the plane containing the axis of the front pivot portion and the axis of the rear pivot portion is allowed from the viewpoint of suppression of vibrations of the power unit.
Furthermore, since the output shaft is located below the cylinder portion so as to be vertically overlapped, the vertical and longitudinal sizes of the power unit can be reduced, so that the power unit can be made compact.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.